In the decades of 1950 and 1960 there was a change of practice taking place in electric power utilities' high voltage power transmission systems. The high voltage circuit breakers were being changed from the old, slower acting and less powerful dead tank "bulk oil" circuit breakers, to live tank, air blast breakers of higher capability. These required separate, free standing current transformers, to perform the function of the Bushing Type current transformers previously located within the oil circuit breaker's tanks. In the general the current transformers used were of conventional insulation design; using quantities of kraft paper tape applied to form the high voltage internal insulation; suitably dried, and impregnated with insulating mineral oil, according to well known practices. This insulation system however has a finite life; depending on the design practices, operating temperatures, and insulation preservation systems used by the original makers. End-of life of these units may be signalled by an internal insulation failure, followed by as powerful as electric current as the system can supply, flowing from the high voltage conductors to ground within the unit. The result is often a violent explosion, followed by a fire as result of ignition of the insulating mineral oil which escapes from the damaged unit. The results of such a failure can be extremely damaging to surrounding equipment and hazardous to near-by personnel. It is extremely difficult to predict by practical means, when such a failure is likely to happen.
At the time of development of early North American air blast circuit breakers, this hazard was foreseen. To avoid it, a special design of gas insulated Current Transformer was developed, using Sulpher Hexafluoride (SF.sub.6) gas under modest pressure as an insulating medium, according to the teaching of U.S. Pat. No. 2,261,671, issued Nov. 4, 1941. These units were completely fire-proof and were considered to be non-deteriorating and with minimal explosion risk. The reduced risk of explosion in the event of an internal fault is due to the fact that an electric arc occurring in the gas causes a more gradual and many times lower rise of pressure within the unit, than that which occurs in a conventional oil-and-paper insulated transformer. In the latter, the electric arc being of extremely high temperature, decomposes the paper and oil explosively, into highly combustible gasses, with an instantaneous volume increase of many orders of magnitude. The physical effect is the same as ignition of a considerable charge of high-explosive material.
The surrounding liquid is an excellent transmission medium for the resultant shock wave. The casing and porcelain insulators comprising the enclosure for the apparatus are blown apart, and the ensuing electric arcs ignite the vapours and liquid oil so liberated and brought into contact with air, which supports the combustion of these substances.
In the gas insulated transformer, on the other hand, the arc is passing through material which is already gaseous. It may be dissociated by the heat of the arc into other gaseous materials, and it will be increased in volume by the heating effects of the arc itself. The resulting rise of pressure, in the vessel which comprises the enclosure of the apparatus is much more gradual and orders of magnitude lower in severity than the previously described event, in a conventional oil-and-paper insulated transformer.
Many gas insulated high voltage current transformers were produced in United States and Canada in the period of about 1956 to 1965 and have given excellent service. Their initial costs were high however, compared to the competing oil-filled units, and manufacture in North America ceased about 1970. Oil filled current transformers continued to supply the needs of the electric utilities.
In recent years there has been increasing concern among electric power utilities because of the number of explosive failures as described above, occurring among these oil-filled units. Some utilities have now specified a new acceptance test on replacement current transformers, to demonstrate that in the event of an internal fault in the unit, and consequent high power electric current there-through; an explosion would not occur. The test current is chosen to be higher than the power system can deliver in practice; and tests must be performed at a special high power test laboratory capable of generating such current with precisely controllable magnitude. No oil-filled current transformers can meet this test requirement without violently bursting. Gas insulated current transformers with conventional porcelain high voltage insulators, as have been produced in the past, are also incapable of meeting these test requirements. In practice, the procelain is fractured and the normal operating pressure contained within the unit is sufficient to hurl the shattered fragments outward with considerable force. Previous attempts to construct an "explosionproof" current transformer using the highly desirable high voltage porcelain insulators have been unsuccessful.
It has been known practice to replace the porcelain insulator by a composite insulator of resinous material, reinforced with high-strength fibrous material, such as fibreglass. Thus the bursting of the procelain insulator is avoided. However, this composite insulator has several disadvantages. For example, it is not as homogeneus or as weather-resistant as the porcelain it is replacing, and must be protected from weather by some external shield or covering. It is also much more permeable to moisture, and can allow the external humidity, always present in the air around it, to pass through and build up to dangerous levels within the said apparatus; where it can cause an internal fault to occur by condensation upon the internal insulating surfaces of the apparatus. The use of these composite insulators to replace the main porcelain insulator is therefore undesirable.